FIG. 1 of the accompanying drawings illustrates the stack structure of a typical liquid crystal display (LCD) module of small size, for example for a mobile phone or PDA device. The display 1 includes a flat transmissive spatial light modulator (SLM) in the form of an LCD panel having input and output polarisers 12 on its bottom and top sides. The rest of the structure is generally regarded as the backlight system, as follows. A light source (for example an LED or Laser) emits light, which is coupled into a light guide and distributed across the back of the display by way of total internal reflection (TIR) in such a way that if no scattering structures were present the light would travel until it reached the end of the light guide. Within the light guide there are multiple scattering structures that extract the light from the light guide to illuminate the LCD panel by disrupting the TIR conditions at the surface of the light guide on which they are located, hence allowing the light to pass through the air light guide interface. These scattering features may be located on either the top or bottom major light guide surfaces. The density of the light scattering features may increase with distance from the light source to maintain a uniform rate of extraction of the light along the length of the light guide. As light is extracted both down and up from the light guide, a reflecting film is placed beneath the light guide to improve the efficiency of the backlight. There are also some optical films between the light guide and the LCD panel, placed to give better illumination uniformity over the display area and to enhance brightness within a given viewing angle range. These films typically consist of diffuser layers and prism films that enhance the central brightness of the backlight.
The prism films typically consist of lenticular lines of triangular cross section on a thin sheet. The triangle cross section is typically an isosceles right angle triangle with the right angle on the apex of the cross section. Two orthogonal prism films between diffusers are typically used in a backlight.
The operation of this prism film is to collimate and recycle light back to the reflector. Light that reaches the prism film at a high angle in a direction substantially perpendicular to the prism lines is reflected closer to the normal. Light at lower angles is reflected by total internal reflection (TIR) back though the diffuser and lightguide to the reflector and reflected back more diffuse, to the prism sheets. The result is an improvement in central brightness of the backlight by redistribution of light from high angle into the center.
There is also an interest in collimated backlights, where the light is distributed about the normal to the backlight and has a significant percentage between 5 and 10 degrees or less to this normal. Collimated light of this form is useful in improving the efficiency of liquid crystal displays. The operation of simple prism films is insufficient to give this level of collimation on their own.
This collimation can be achieved through control of the extraction features. The typical form of extraction features involves “roughening” of the surface in some manner to disrupt total internal reflection (TIR) in the lightguide. The extraction in this case produces light that is emitted at a high angle to the lightguide normal.
There are many types of extraction features that can control the angle of extraction, for example U.S. Pat. No. 6,786,613 (Minebea) describes wedge shaped extraction features that extract light in a more vertical direction.
Light extracted in a relatively collimated manner cannot be used with prism films because such light is recycled and the operation of these prism films serves to reduce the collimation angle and reduce the overall light throughput.
Other known methods of improving brightness and collimation are described below.
U.S. Pat. No. 7,286,280 (The University of British Columbia) describes a film with prismatic structures with rounded edges that can be deposited over brightness enhancement films (BEFs) to further enhance brightness gain while maintaining large viewing angle.
U.S. Pat. No. 6,570,710 (Reflexite Corporation) describes a dual side BEF film. A prismatic BEF like structure is formed on the top of the film while a sub-wavelength moth-eye shaped grating is formed on the bottom of the film. The grooves of the moth-eye grating are aligned vertically to the prism of the BEFs eliminating the chromatic non-uniformities at large viewing angles that BEFs usually exhibit.
U.S. Pat. No. 7,128,443 (Philips Electronics) describes a BEF supporting film with reflecting walls formed on its top surface. The slope of the walls is adjustable with respect to the film normal which has an effect to the cone of light that reaches the BEFs and so to the collimation of light. In this way the central brightness and the viewing angle of the display can be tuned.